1. Field of Invention
The present invention relates generally to the mechanism of heat and mass transfer and more particularly to a method and apparatus for enhancing the natural instabilities in the boundary layer and free shear layer of a stream of fluid that flows along a solid surface.
2. Description of the State of Art
Heating, ventilation, and air conditioning (HVAC) systems introduce or inject air into a room through narrow, elongated nozzles or air diffusers to produce a substantially planar (narrow, elongated) jet or free jet of air. The emerging jet is the primary motion in the air diffusion process. However, as the jet entrains mass from the ambient room air it induces a secondary room air motion because the entrained mass is replaced by adjacent air. Therefore, the secondary room air motion is the mechanism by which ambient air, which carries potentially harmful pollutants, is brought to the jet where it is diluted by mixing and eventually removed in the return air. Farrington in U.S. Pat. No. 5,338,254 disclosed that the mixing properties of a free jet may be increased by imparting periodic pressure pulsations to an air flow upstream of the jet outlet so that the frequency of the pulsations matched the natural characteristic frequency of a turbulence in the free jet emerging from an air diffuser. This process has been found to be an effective way to achieve a high degree of mixing, of the incoming air with the ambient air in a room.
In addition to the mixing, entrainment, and spreading properties that exist for a free jet, heat and mass transport properties exist for another flow phenomena, referred to as a wall jet. The term "wall jet", as described by Bakke P., J. Fluid Mechanics, 2:467 (1957), refers to the flow field created "when a jet, consisting of a fluid similar to that of its surroundings, impinges on a plane surface and spreads out over the surface."
There are three basic means by which heat transfer can occur: convection, conduction, and radiation. Convection, refers to the transfer of heat between a body and a fluid, and takes place primarily by interchanging the physical position of molecules. This is the primary means of air drying. Whereas, the transfer of heat by conduction involves the interchange of kinetic energy between molecules without displacing molecules. Obviously, convective heat transfer involves flow phenomena, such that, heat transfer is governed by the fluid-flow characteristics 10, of the system, as shown in FIG. 1.
In general as a fluid F or collectively a jet J, emerges from outlet 12, a free shear layer 14 develops at the free edge 16 of the jet J, and a boundary layer 18 develops at the plane surface S, such as, the surface of a paper, textile, wall, window, or ceiling. Each of these layers 14 and 18 grow and at some point P downstream they meet. The region near outlet 12, where the two viscous layers 14 and 18 have not yet propagated all the way across the flow in the transverse direction, is the inviscid or potential core region 20 where local velocities between the free shear layer 14 and boundary layer 18 are unaffected by viscosity.
In free shear layer 14, turbulent structures 22 form due to instabilities that result from the steep velocity gradients and associated viscous effects. These turbulent structures 22 form an array of large-scale vortices which entrain mass. Farther downstream, the vortical structures interact by pairing, coalescing, and tearing and are eventually broken down by viscous diffusion until complete mixing has occurred.
Within boundary layer 18, fluid molecules that come in contact with surface S remain essentially stationary (with respect to surface S), a condition referred to as no-slip, while molecules in jet J move with the velocity of jet J. Between these two extremes, layers of molecules move at intermediate velocities as the fluid shears (molecules slipping past one another). This region of shear as a whole is known as the boundary layer. At low velocities, each individual layer of molecules present in boundary layer 18 slips past the adjacent layers without significant interchange of molecules between layers. Under this condition boundary layer 18 is described as laminar. At higher velocities, boundary layer 18 becomes turbulent, although a portion of it known as the laminar sublayer 24, remains in the laminar regime.
Mass transfer, just as heat transfer, is also dependent on the flow characteristics of the air. Through laminar layers, mass transfer is controlled by molecular diffusion, while through turbulent portions of the boundary layer, it is controlled by eddy or convective, diffusion. Molecular diffusion, which involves the interchange of position, molecule by molecule, is a relatively slow process. On the other hand, eddy diffusion involves a rapid relocation of molecules by turbulent motion. Thus, as with heat transfer, laminar sublayer 24 is a critical element of mass transfer.
The efficiency and overall effectiveness of heat and mass transport are properties of significant importance in many industries concerned with the removal of moisture from a substance or the cooling of a substance. For example, wall jets are utilized in the automotive industry, for purposes of defogging automobile windshields, in the paper and textile industries for purposes of evaporating moisture and in the glass and metal industries for cooling sheets or processed glass and metal.
For example, paper, to be useful, normally requires a moisture content of less than 0.1 lb water/lb paper. However, after pressing, the sheet still contains from 1 to 3 lb water/lb finished paper, depending on the particular machine and product. Since no particular method of direct liquid extraction has been developed to reduce the moisture content below the level of 1 lb/lb finished paper, it is necessary to resort to the relatively expensive process of evaporation. One mechanism commonly utilized in the evaporation process is air drying. In the air drying process, air serves as the medium for both heat and mass transfer. The heat for evaporation is applied to the sheet by convective heat transfer from the air surrounding the sheet, then evaporated moisture diffuses into this air and is ultimately carried away by it.
In air drying, the sheet can be considered as the solid, while air is the flowing fluid. Therefore, "[f]or overall convection heat transfer in air drying, heat flows through the laminar sublayer [24] by conduction, while through the turbulent portion of the boundary layer [18] it flows primarily by convection. Since conduction heat transfer through air is very inefficient (air being one of the best insulators known to man) while heat transfer by air convection is much more efficient, laminar sublayer [24] controls the overall rate of heat transfer. Thus the thickness of laminar sublayer [24] is all-important to efficient heat transfer" (emphasis added). See, Coveney D., et at., Paper Making and Paperboard Making, 2nd ed., 3:405-551, 464 (1970).
"The need to improve the efficiency of air drying has led to the use of high-velocity and high-temperature air jets impinging directly on the sheet. By impinging the air at high velocity against the sheet, the boundary-layer thickness is minimized, thereby improving both heat and mass transfer . . . All impinging air dryers use air jets generally perpendicular to the web." Id. at 467. High velocity, high-temperature impingement air drying requires large power outputs and can not be used on lighter grades of paper. If the boundary-layer thickness could be minimized, without the requirement of high-velocity, high-temperature impingement, heat and mass transport would be enhanced leading to a more efficient and cost effective means of air drying.